Thread: diff ratio change

It didn't work with 4.12 and 33s the. When I changed to standard ratio it worked again the cruise control

Originally Posted by shack

It's quite possible there was an issue with the actual cruise control itself, bigger tyres cannot make any difference to the speed the abs sensors pick up.

Originally Posted by Slunnie

The SLABS generates and sends a speed signal that is taken from the 4 ABS sensors. When you change the diff ratio, the driveline speed stops matching what it expects to see at the axle because of the diff change. If you use a speedo correction unit on the speed output signal at the SLABS computer it will correct this. BUT when you are doing this, you're not correcting the speed of the car, you are correcting what it should see because of the diff ratio change - this is important. So, to gain cruise control back again, correct from 3.54 diffs to 4.12 diffs. If you're running 285/75-16 (33's) then your speedo will be accurate. To do this you will need to use a pretty specific speedo correction device that needs to pass through the baseline diagnostic frequency, which most do not do. See this article I wrote about 17 years ago for fitting and a SCU that works.

Truspeed speedo correction installation - www.slunnie.com

I think the HunterDi one is the only speedo corrector easily available now that actually works on the D2 as it allows that base frequency through to the BCU

Relatively pricey but worth it. The only thing that has slightly altered is the lockup speed in 4th, it's now 85km/h instead of closer to an indicated 80 with stock rubber, both with a stock map.
3rd is as low as 60 now in the right situations, and HDC is spot on at 7km/h

Originally Posted by dieselcarb

This was an interesting read just to clarify as it says you had an auto did you have to do this to get everything working like crues control ect? or was it just to correct speedo?

many thanks

The auto doesn’t have the CC problem, you can do whatever you like and everything will continue to work, although as Rick130 says, the torque converter lockup points may shift - that was my experience also and I believe this to be correct, although others think there is no electrical connection between the auto controller and the speed output from the SLABS. So, on the auto, its really about correcting the speedo to the tyre size, eg 29’s to 33’s.

The manual, if you want the CC to work, you need to correct the speedo to suit the driveline speedo. Eg 3.54 to 4.12 and/or 1.2 to 1.4 Transfer case high range - then everything will work.

In the manual, if you’re running stock driveline ratios and everything works like the CC but you want to correct the speedo still for bigger tyres and not lose the CC, I understand you can pick up the speedo signal where it goes into the dash and so only the dash is altered. I have not done this and can not personally verify, but I understand this is ok to do - I would verify this with someone who has done it and make sure the dash odometer doesn’t become conflicted with the BCU odometer and start flashing because it thinks it has been tampered with.

So I bought the speed kit now from hunter so I just follow your online tutorial and then it's all good? Is it adjustable or just install and leave

Originally Posted by Slunnie

The auto doesn’t have the CC problem, you can do whatever you like and everything will continue to work, although as Rick130 says, the torque converter lockup points may shift - that was my experience also and I believe this to be correct, although others think there is no electrical connection between the auto controller and the speed output from the SLABS. So, on the auto, its really about correcting the speedo to the tyre size, eg 29’s to 33’s.

The manual, if you want the CC to work, you need to correct the speedo to suit the driveline speedo. Eg 3.54 to 4.12 and/or 1.2 to 1.4 Transfer case high range - then everything will work.

In the manual, if you’re running stock driveline ratios and everything works like the CC but you want to correct the speedo still for bigger tyres and not lose the CC, I understand you can pick up the speedo signal where it goes into the dash and so only the dash is altered. I have not done this and can not personally verify, but I understand this is ok to do - I would verify this with someone who has done it and make sure the dash odometer doesn’t become conflicted with the BCU odometer and start flashing because it thinks it has been tampered with.

Originally Posted by Kaaaiju

So I bought the speed kit now from hunter so I just follow your online tutorial and then it's all good? Is it adjustable or just install and leave

I havent installed a Hunter DI unit, but it should be exactly the same and you will need to calibrate it - just use a GPS unit and I've calibrated at 100km/h which reads slightly fast at 60km/h.

Originally Posted by Kaaaiju

So I bought the speed kit now from hunter so I just follow your online tutorial and then it's all good? Is it adjustable or just install and leave

Originally Posted by Slunnie

I havent installed a Hunter DI unit, but it should be exactly the same and you will need to calibrate it - just use a GPS unit and I've calibrated at 100km/h which reads slightly fast at 60km/h.

What Slunnie said.
It comes with a comprehensive ratio chart to get the correct output ratio for your current tyres.
I just used the GPS on my phone.

Originally Posted by Kaaaiju

It didn't work with 4.12 and 33s the. When I changed to standard ratio it worked again the cruise control

Just to clarify, with standard diff ratios it will always work fine irrespective of the wheel size, so 33s are fine, but of course in your case the issue was the 4.12 gearing. That would need correcting through the engine mapping.

Yeah it was 4.12 but how can it be mapped out? Some here are saying use the speed correct

Originally Posted by shack

Just to clarify, with standard diff ratios it will always work fine irrespective of the wheel size, so 33s are fine, but of course in your case the issue was the 4.12 gearing. That would need correcting through the engine mapping.